The present invention is concerned with a light-emitting device coupled to an optical fiber. The light-emitting device is an essentially Lambertian source of emission on the surface of a semiconductor crystal mounted on a base. The receiving end of at least one optical fiber is held in position in front of the emitter with the aid of two flexible rings fitted in a cavity in a metal sleeve. The rings, sleeve, and base together form an air-tight protective container having an axial passage through which the optical fiber passes.
It is known in the field of telecommunications that certain electrical coaxial connections can be replaced by optical connections with the aid of optical fibers placed in a suitable envelope and interconnecting a light-emitting source and a light-receiver. These optical fibers, consisting of a cylindrical core and a sheath with a refractive index different from that of the core, in fact offer the advantage of being insensitive to unwanted external interference. They also have a lower attenuation coefficient than traditional coaxial cables, so that they can be used in high frequency ranges on the order of gigahertz.
A single fiber is generally used to form an optical connection. It is either single-mode when it operates with very directional and coherent source of radiation, it is multimode when the source is not monochromatic. Nevertheless, in certain cases, particularly for reasons of security in transmission, a number of fibers may be grouped into a bundle in an envelope to form an optical connection.
The use of an optical fiber or a bundle of optical fibers nevertheless presents a certain number of difficulties, particularly with regard to its or their coupling to the source of emission.
It is known that the angle of acceptance of an optical fiber does not exceed 10.degree. generally. Consequently, if the source of emission is highly directional, such as a laser, its coupling with the fiber can be satisfactorily effected. However, if the source of emission radiates into space essentially according to Lambert's law (a Lambertian source), only a small quantity of the light emitted by the source is able to enter the fiber.
In most applications, the sources of emission used are light-emitting diodes formed on the surface of a semiconductor crystal. These diodes transmit light from that surface either directly to the outside, or indirectly after passing through the material composing the crystal. The light is emitted according to a law comparable to Lambert's law. to ensure optimum transmission of data, it is therefore desirable to maximizing the light entering the fiber, or the bundle of fibers, and therefore to exercise the greatest care in centering the source of emission and fixing its distance in relation to the receiving face of the fiber or bundle of fibers. The severity of this problem increases as the cross-section of the receiving face of the fiber or of the bundle of fiber becomes smaller than the surface of the source of emission.
The mutual centering and the maintenance in position of the source of emission and of the fiber or the bundle of fibers may be effected by a well-known procedure which consists principally in fixing the source of emission on a base and applying a radial pressure to the fiber or bundle of fibers with the aid of, for example, at least one flexible and elastic ring.
The best known means of applying this radial pressure to the ring are relatively varied. One of the simplest of these known means consists of inserting the ring in a nondeformable housing of configuration and dimensions essentially identical to the configuration and external dimensions of the ring. By now applying to the ring a pressure in the longitudinal direction, since the housing is nondeformable the ring necessarily undergoes a centripetal deformation.
This deformation must be sufficient to keep the optical fiber in place but must not cause the fiber to crack or break.
This clamping problem, which is complicated with even a single ring, becomes more difficult to resolve when there are several rings placed at the end of the fiber to ensure that it is optimally centered.
In fact, the existence of several clamping points means that the radial pressure applied to the optical fiber should be equally distributed between them so as to avoid internal deformation of the crystal structure of the fiber or even its breakage. This problem becomes more acute as the diameter of the fiber becomes smaller.
A solution recently proposed by the applicants in French patent application No. 79 11 451, consists in clamping the end of the optical fiber in a rigid sleeve which is itself subjected to the radial pressure exerted by the flexible and elastic rings.
This solution requires some care and precision in the positioning of the sleeve and in the insertion of the assembly in the rings, each previously inserted in a housing hollowed out inside a metal sheath.
Furthermore, certain applications may necessitate hermetically sealing the device against ambient atmospheric or chemical agents. However, a sequence of rings or tubes of various natures means as many successive interfaces which, in the event of a fault, however slight, may contribute to the occurrence of leaks.